There currently are two basic methods which are conventionally utilized for coating the leads of semiconductor devices packaged on a lead frame with a soft material, such as a solder material. These two methods involve a solder plating method and a solder immersing (i.e. dipping) method.
In the immersion method, there is no need for complementary electric equipment. Rather, the metal part to be plated is coated by simply immersing the part or workpiece in the solder material. This method is advantageous due to its simplicity. However, in the case of solder coating of lead frames, there is a significant problem, namely how to eliminate the thermalshock imposed on the electronic device inside the plastic package when the device mounted on the lead frame is immersed in the bath of hot metal.
Accordingly, when it is desired to solder coat the leads of semiconductor devices using the immersion method, the semiconductor package is separated from the lead frame so that each package is then individually handled and is dipped into the solder bath in such a way that only the lead section is immersed and coated. However, since each semiconductor device normally has a plurality of lead sections projecting not only from a first pair of opposite sides of the plastic package, but also from a second pair of opposite sides of the package, this hence requires that each package be individually handled and controlled so as to permit immersion of solely a single lead section, followed by manipulation of the package to permit immersion of a further lead section. This hence requires significant manipulation of the package, followed by immersion several times, depending upon the number of lead sections provided on the package. While this procedure avoids immersion of the plastic package into the hot solder bath so as to avoid the thermalshock problem, nevertheless the procedure is obviously complex and time consuming in view of the multiple handling steps required, and the requirement that such steps be accomplished with respect to each single package.
The dipping or immersion method is hence inefficient and unsatisfactory for use with semiconductor devices which mount on lead frames and which have lead sections projecting outwardly from two or more sides of the plastic package.
As an alternative method, the Assignee of the present invention has developed a solder coating apparatus which uses a pair of coating rolls between which the lead frame is fed before the semiconductor packages are cut from the lead frame. With this apparatus, the rolls dip into a solder bath so as to pick up a coating of solder thereon, and then transfer the solder to the leads associated with the lead frame as the lead frame moves through the nip between the coating rolls. This apparatus permits automatic and substantially continuous coating of the leads associated with the lead frame.
In the aforementioned roll coating apparatus, if the packages mounted on the lead frame have lead sections projecting outwardly only from opposite sides which extend transversely relative to the elongated direction of the lead frame, then the leads associated with the plurality of packages on the lead frame can be coated by a single pass between the rolls. However, when the packages are of the type having leads sections which project from transversely oriented sides of the plastic package, then such lead frames must be passed through two types of roll coating devices, with one such device involving multiple aligned pairs of rolls so as to permit the lead frame to pass therebetween in a direction perpendicular to its elongated direction. While this prior apparatus has advantageous operating features in that it permits the lead sections of multiple packages to be coated while still attached to the lead frame, nevertheless such apparatus still does not possess the desired simplicity.
Accordingly, it is an object of this invention to provide an improved apparatus and method for solder coating, particularly for solder coating of multiple lead sections associated with semiconductor devices which are united to a common lead frame. This apparatus permits more efficient handling of the lead frames and coating of the lead sections.
According to the present invention, a lead frame having thereon a plurality of semiconductor devices is prepared by immersing it in a flux solution to activate the lead frame surface and to remove oxide film. The lead frame is then transferred to a coating block unit, and the lead frame is moved along a lateral passage which extend through the coating block unit. During movement through the passage, the lead sections of the lead frame are coated on both the upper and lower surfaces thereof with liquid solder material which ascends by capillary action along a slit which projects upwardly from a liquid solder bath and communicates with the passage. The solder in the slit, where it communicates with the passage, is continually replenish by capillary action from the bath as the solder coats the lead sections associated with the lead frame as the lead frame moves through the passage. The liquid solder bath has an inert gas discharge pipe therein. The feed slit for the solder is of microwidth, preferably in the range of between 0.1 and 0.5 mm, to facilitate the feeding of solder upwardly through the slit by capillary action.
Described in greater detail, the continuous solder coating apparatus of this invention includes: a flux containment device defining a flux bath in which the lead frame is immersed; a liquid solder bath which has an inert gas discharge pipe therein; a solder coating block having its lower end immersed in the solder bath and having a number of capillary slits feeding upwardly from the bath to a lead frame transfer passage, which slits correspond in number to the number of lead sections which project in the lengthwise direction of the lead frame; a lateral feeding device which feeds the lead frame laterally (i.e., sidewardly) through the aforementioned coating block so that all of the lead sections which project longitudinally of the lead frame are simultaneously coated on both upper and lower surfaces thereof due to the upward capillary feed of solder through the slits; a flux coating device which applies a flux solution on both the upper and lower surfaces of the laterally projecting lead sections by two pairs of brush wheels, which lower wheels are immersed in the flux bath; a second liquid solder coating bath having an inert gas discharge pipe therein; and a second solder coating block which has a lower end immersed in the second bath and which is provided with a guide passage for permitting longitudinal movement of the lead frame therethrough, which latter block has two capillary slits which project upwardly from the bath and communicate with the passage adjacent opposite sides for coating the lateral lead sections as the lead frame moves longitudinally through the passage.
In the solder coating block of the present invention, there is provided opposed coating and guide bodies which cooperate to define the passage through which the lead frame passes, which passage is formed by grooves in the upper and lower coating body. The lower coating body also includes a groove to receive and to dispose of surplus liquid solder.
By providing the solder bath with an inert gas discharge therein, this is effective for preventing oxidation of the solder in the bath. Further, the inert gas becomes entrained in the solder, and ascends with the liquid solder which ascends upwardly through the slit by capillary action, so as to prevent oxidation.
With the solder coating apparatus of the present invention, both upper and lower surfaces of the lead sections can be efficiently and uniformly coated with solder simultaneously, while feeding solder solely upwardly by capillary action through small feed slits. This permits controlled coating of both sides of the lead sections so as to provide for desired and uniform coating thicknesses while preventing the build up or bridging of solder between the individual leads of the lead section. This also prevents the application of hot solder to the plastic package. This permits a plurality of semiconductor devices as mounted on a single lead frame to be continuously and simultaneously processed. Further, when the semiconductor device involves a conventional square plastic package having lead sections projecting outwardly from all four sides thereof, the lead sections can be conveniently and efficiently coated by effecting coating in sequential steps, with one step involving movement of the lead frame in a lateral direction through a coating block, and the other step involving movement of the lead frame in a longitudinal direction through a coating block.
Other objects and purposes of the invention will be apparent to persons familiar with methods and apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.